...nobody said anything about this, so i set up move-if-change and
timestamp files in usr.bin/egcs/commmon/Makefile. i used the standard
s-* timestamp file-names, as in egcs's Makefile.in.

This avoids touching the machine-generated insn-*.[ch] files if they
don't acutally change. A `make build UPDATE=1' will relink the
generators. Without the move-if-change logic, that would force a
re-make of the insn-*.h and rtl files, which in turn forces a rebuild
of just about everything.

I didnt do any `move-if-change' style processing outside of
egcs/common (e.g., yacc-generated files). Feel free to add it.

(PS: the latest egcs import builds on pmax, but i havent dared
rebuild the world with it yet...)